Just like when you GET to work, you should have a routine for when you get to LEAVE work.

Clean Up.  Meaning your desk and anything in the kitchen you're responsible for.  That way you'll feel like you have a fresh start when you come in the next day.

Say Goodbye.  It only takes a second, and helps you stay friendly with your co-workers.  Nobody likes the person who just gets up and leaves every day without saying anything.  Plus, it's good to check in with your boss to make sure nothing else needs to be done.  If you are the boss, then saying hello and goodbye is even more important.

Don't Take Your Work Stress with You.  Chances are you have plenty of stress to deal with at home too, so do your best to forget about work once you leave the office.  If you have a desk job, try turning off the email alerts on your cell phone so you won't check them all night.  Just make sure you tell your co-workers you're doing it, so they know they'll have to call if they need to get in touch. 

Look into what else you can do to finish up your day here.